In general, a spark plug has a metal shell, an insulator, a center electrode insulatedly held in the metal shell via the insulator and a ground electrode joined to the metal shell. The spark plug generates a spark discharge between the ground electrode and the center electrode for ignition of an air-fuel mixture in a combustion chamber of an internal combustion engine. However, the spark plug fails to generate a spark discharge when the voltage applied becomes lower than a required voltage (called a spark discharge voltage) with decrease in insulation resistance due to the deposition of carbon on a surface of the insulator by incomplete combustion or the like. There have thus been developed various techniques for preventing fouling of the insulator due to the deposition of carbon.
For example, Japanese Laid-Open Patent Publication No. 2016-4730 discloses a technique in which a protrusion is formed on the insulator so as to protrude in a direction intersecting an axis of the spark plug. In the technique of Japanese Laid-Open Patent Publication No. 2016-4730, a carbon deposit on the protrusion provides a conductive path between the center electrode and the metal shell so that a discharge occurs in an air gap along the conductive path. By this discharge, carbon deposited on the insulator is burned away.
Against the above technical background, there has been a demand to ensure the fouling resistance of the spark plug with a simpler configuration.
The present invention has been made to satisfy such a demand. An advantage of the present invention is a spark plug capable of attaining fouling resistance with a simple configuration.
In accordance with a first aspect of the present invention, there is provided a spark plug comprising: a center electrode extending along an axis from front to rear; a cylindrical insulator having formed therein along the axis an axial hole in which the center electrode is arranged, the insulator including a step portion formed on an outer circumferential surface thereof and having a diameter increasing from a front end side to a rear end side; a cylindrical metal shell arranged radially outside the insulator, the metal shell including a shelf portion formed on an inner circumferential surface thereof and facing the step portion in a direction of the axis; and a ground electrode joined to the metal shell and facing the center electrode, wherein the insulator includes a front end portion located frontward of the step portion; wherein an outer circumferential surface of the front end portion has an arithmetic average roughness of 0.5 μm or smaller in a circumferential direction; and wherein a recess is formed with a depth of 3 to 20 μm in at least part of an end surface and the outer circumferential surface of the front end portion so as to extend from the front toward the rear.
In a spark plug according to the first aspect of the invention, the outer circumferential surface of the front end portion of the insulator, which is located frontward of the step portion of the insulator, has an arithmetic average roughness of 0.5 μm or smaller in the circumferential direction; and the recess is formed with a depth of 3 to 20 μm in at least part of the end surface and outer circumferential surface of the front end portion. With this structure, carbon is unlikely to be deposited onto the end surface and outer circumferential surface of the front end portion but is easily deposited in the recess. As the carbon deposited in the recess provides a conductive path, a discharge is generated along the conductive path so that carbon deposits on the insulator can be burned away by the discharge. Therefore, the spark plug ensures the fouling resistance of the insulator with a simple configuration.
In accordance with a second aspect of the present invention, there is provided a spark plug as described above, wherein the depth of the recess is 5 to 10 μm. In this case, it is possible to more easily cause deposition of carbon in the recess while ensuring the strength of the front end portion of the insulator. The spark plug thus achieves improved fouling resistance in addition to the effects of the first aspect of the invention.
In accordance with a third aspect of the present invention, there is provided a spark plug as described above, wherein a width of the recess in the circumferential direction is 3 to 200 μm. In this case, it is possible to more easily cause deposition of carbon in the recess. The spark plug thus achieves improved fouling resistance in addition to the effects described above with respect to the first and second aspects of the invention.
In accordance with a fourth aspect of the present invention, there is provided a spark plug as described above, wherein a length of the recess in the direction of the axis is 0.1 to 20 mm. In this case, it is possible to easily provide the conductive path for burning away of carbon deposits. The spark plug thus achieves improved fouling resistance in addition to the effects described above with respect to the first through third aspects of the invention.
In accordance with a fifth aspect of the present invention, there is provided a spark plug as described above, wherein two to eight recesses are formed in the front end portion at positions apart from each other in the circumferential direction. In this case, there are provided a plurality of conductive paths by deposition of carbon in the two to eight recesses so that it is possible to easily generate a discharge for burning away of carbon deposits. The spark plug thus achieves improved fouling resistance in addition to the effects described above with respect to the first through fourth aspects of the invention.
In accordance with a sixth aspect of the present invention, there is provided a spark plug as described above, wherein the recesses are equally spaced apart from each other in the circumferential direction. In this case, the recesses are arranged in all directions in a state that the spark plug is mounted to an internal combustion engine. The spark plug thus prevents variations in fouling resistance depending on the orientation of the insulator on the internal combustion engine, in addition to achieving the effect of the fifth aspect of the invention.
In accordance with a seventh aspect of the present invention, there is provided a spark plug as described above, wherein assuming, in a cross section perpendicular to the axis, a first imaginary straight line passing through the axis and a second imaginary straight line passing through the axis and intersecting the first imaginary line at a right angle, a first region of the front end portion overlapping the first imaginary straight line is greater in length than a second region of the front end portion overlapping the second imaginary straight line; and the recess is located on the front end portion within the range of ±15° from the first region. In this case, it is possible to secure the thickness of the part of the front end portion in which the recess is formed and suppress the influence of the recess on the strength and insulating properties of the front end portion. Thus, the spark plug ensures the strength and insulating properties of the front end portion in addition to achieving the effects described above with respect to the first through sixth aspects of the invention.
In accordance with an eighth aspect of the present invention, there is provided a spark plug as described above, wherein a value of the length of the second region being divided by the length of the first region is in a range of 0.7 to 0.96. In this case, the spark plug ensures withstand voltage and prevents penetration breakage of the front end portion caused starting from the recess by the applied voltage, in addition to achieving the effects of the seventh aspect of the invention.
In accordance with a ninth aspect of the present invention, there is provided a spark plug as described above, wherein the recess is located at a side opposite from the ground electrode with the center electrode interposed therebetween when viewed in the direction of the axis. As compared to the ground electrode-side, there is a wide space for the growth of a flame kernel on the side opposite from the ground electrode so that a large flame can be developed for burning away of the carbon deposited in the recess. As it is possible to burn away carbon deposits over a wide area on the front end portion, the spark plug achieves improved fouling resistance in addition to the effects described above with respect to the first through eighth aspects of the invention.
In accordance with a tenth aspect of the present invention, there is provided a spark plug as described above, wherein the insulator includes a protruding portion protruding radially outwardly from the outer circumferential surface thereof at a position rearward of the step portion; the metal shell has an engaged part formed on the outer circumferential surface thereof at a position rearward of the shelf portion; and the protruding portion has an engaging part that engages with the engaged part in the circumferential direction. The recess of the insulator is positioned relative to the metal shell by circumferential engagement of the engaging part with the engaged part. Thus, the spark plug achieves easy positioning of the recess relative to the metal shell in addition to the effects described above with respect to the first through ninth aspects of the invention.